Are you ready for one final look at the brightest comet for over a decade? After wowing sky-watchers in September and October, comet C/2023 A3 (Tsuchinshan-ATLAS) is fading out in the night sky, but a virtual observatory will make it visible to all on YouTube for one final time today.
It's a long-period comet made from ice, dust and rocks that orbit the sun. They're often called "dirty snowballs." Comet Tsuchinshan-ATLAS was discovered in January 2023 by astronomers at China's Tsuchinshan (Purple Mountain) Observatory and South Africa's Asteroid Terrestrial-impact Last Alert System (ATLAS) telescope.
This December marks the last visible days of comet A3 before it heads far away from the sun and fades from view. Now, in the constellation Aquila, Comet Tsuchinshan-ATLAS is tonight 237 million miles (381 million kilometers) from Earth. It's now shining at a magnitude of about +9, significantly beyond the threshold of the human naked eye.
The Virtual Telescope Project will broadcast views of comet Tsuchinshan-ATLAS as it heads away from Earth. Its live feed is scheduled for 17:00 UTC (midday EST) on Dec. 23.
"After its amazing show last October, comet C/2023 A3 Tsuchinshan-ATLAS has left so many fond memories to all those loving the wonders of the night sky," said Gianluca Masi at The Virtual Telescope Project, in an email. "While the comet is now a telescopic object, it is still visible in our sky. We decided to offer a chance to look at this comet once again, perfectly on time with the upcoming Holiday Season!"
Comet Tsuchinshan-ATLAS is from the Oort Cloud, a distant, spherical shell of icy objects surrounding the solar system home to millions of comets. Comet Tsuchinshan-ATLAS appears to loop around the sun only once every 80,000 years. It reached its perihelion in September -- the closest it gets to the sun on that journey. It will return to the Oort Cloud, turning back into the sun in about 40,000 years.
There are dozens of comets in the inner solar system at any one time. Few of them, however, become visible to the naked eye -- or even become a binocular object. So what was so special about comet Tsuchinshan-ATLAS?
It's all about celestial geography. The comet passed between the sun and Earth in late September and October, so light diffracted (bent around) by dust grains in the tail, which were concentrated in the direction of Earth. It's similar to when thin clouds look bright because they're backlit by the sun.